1. Field of the Invention
This invention relates to a wheel-supporting rolling bearing unit with air compressor, which is used for supporting an automobile wheel so that it can rotate freely with respect to the suspension apparatus, and that properly maintains air pressure in the tires of the wheel.
2. Description of the Prior Art
There is a proper value for the air-pressure in the tires of automobile wheels, however, this air pressure gradually decreases due to leaking and varies due to the outside air temperature. In other words, the gas barrier of the rubber material of the tires is not perfect, and there is also a possibility that leaking from the valve section of the air-supply section may become greater than leaking through the rubber material. Moreover, as is clear from Boyle's Law and Charles's Law, even when leakage is ignored, the air pressure fluctuates 10% or more in the summer and winter, even though the rise in temperature due to movement of the vehicle is ignored, and when taking into consideration the rise in temperature due to the vehicles movement, the air pressure may fluctuate 30% or more. However, except for the case of commercial vehicles such as trucks, busses, taxis and the like, there are very few drivers who constantly check the air pressure.
Taking into consideration this situation, in the case of a typical driver of a private vehicle, it is common at the time of required vehicle inspection to set the air pressure of all of the wheels to a value higher than the proper value. The reason for this is that the air pressure of each of the wheels will decrease somewhat due to leaking that occurs until the next required vehicle inspection, and this is thought to make it more difficult for dangerous conditions such as bursting to occur when driving at high speed even though the driver does not maintain the air pressure. However, in this situation, the air pressure of each of the wheels is set higher than the proper value, which together with making riding comfort of the automobile poor, adversely affects driving stability and brake performance of the automobile. Particularly, this tendency becomes more severe when regular vehicle inspection is performed in a season such as the beginning of spring when the outside temperature begins to increase. In recent years, leakage of air is kept to a minimum by changing the gas used to fill the wheels from air to nitrogen gas that does not contain other gasses such as oxygen; however, it is still difficult to completely eliminate leaking.
Also, in order to improve vehicle performance such as comfort, driving stability and braking, it is preferable that the air pressure be adjusted according the road conditions. In other words, in order to improve the aforementioned vehicle performance when driving on a poor road such as a gravel or dirt road, it is preferable that the air pressure be set lower than when driving on a good paved road. However, even when the air pressure has been set lower for driving on a poor road, when driving particularly at high speed on a good paved road after that, the air pressure must be increased, so it is necessary to fill each of the wheels with air. Moreover, the same problem occurs when considering that the wheels must be filled with air again after the temperature has decreased after lowering the air pressure of the wheels that has greatly increased due to rising temperature when driving during the summer. However, having to stop the car to fill the wheels with air with a separate air compressor every time the road conditions change, or every time the driving conditions or temperature changes is not practical. Therefore, in order to meet the needs described above, it is necessary that each of the wheels be constructed so that they can be filled with air while driving the automobile.
Construction that meets the aforementioned needs is known, for example, the construction disclosed in patent documents 1 to 9. Of the construction disclosed in these patents, the construction disclosed in patent documents 1 to 7 presumes that a supply source for compressed air, such as an air compressor, is built into the vehicle body side. Also, the compressed air that is supplied from this supply source via the suspension apparatus to the inside of the stationary side of the bearing ring, is fed to the rotating side of the bearing ring via a sealed space that is partitioned by a seal ring, and furthermore is supplied to the wheel that is fastened to the rotating side of the bearing ring. Moreover, with the construction that is disclosed in patent document 8, a reciprocating-piston air compressor is fastened to the hub that rotates with the wheels of a large automobile such as a truck in the axial direction of the hub. Furthermore, with the construction disclosed in patent document 9, a reciprocating-piston air compressor is fastened to the hub that rotates with the wheels of a bicycle in the axial direction of the hub. In the case of the construction disclosed in either patent document 8 or patent document 9, compressed air that is output from the air compressor is supplied to the wheels.
Of the construction disclosed in patent documents 1 to 9 described above, in the case of the construction disclosed in patent documents 1 to 7, in order to feed compressed air from the air compressor located on the vehicle body side to the wheels that are fastened to the rotating side of the bearing ring, a complex air-supply path is necessary. In order to always maintain the air pressure in the tires at a proper pressure, it is necessary to maintain the seal of this air-supply path even after use over a long period of time, which results in increased cost.
On the other hand, in the case of the construction disclosed in patent documents 8 and 9, instead of the above problem, a single reciprocating-piston air compressor is located next to the hub, so in order to maintain the necessary amount of compressed air it is necessary to use a large compressor for this air compressor. By placing a somewhat large reciprocating-piston air compressor in the axial direction of the hub in this way, the overall wheel-supporting rolling bearing unit with air compressor becomes large. Also, in the reciprocating-piston air compressor, the force required for moving the piston through one cycle greatly fluctuates, so when driving a single reciprocating-piston air compressor together with the rotation of the hub, the rotation friction of this hub greatly fluctuates through one rotation. From the aspect of maintaining the driving stability of an automobile in which this kind of wheel-supporting rolling bearing unit with air compressor is installed, this kind of condition is not desirable.
[Patent Document 1]
    Japanese Patent No. 2,540,745[Patent Document 2]    Japanese Patent No. 2,940,105[Patent Document 3]    U.S. Pat. No. 5,203,391[Patent Document 4]    Japanese Patent Application Publication No. 2000-255228[Patent Document 5]    US Patent Application Publication No. 2003/0217798[Patent Document 6]    US Patent Application Publication No. 2003/0235358[Patent Document 7]    US Patent Application Publication No. 2004/0005104[Patent Document 8]    Japanese Patent Application Publication No. H11-139118[Patent Document 9]    Japanese Patent Application Publication No. 2004-136797